In the manufacture of internal combustion engine cylinder blocks, automotive heat exchangers such as radiators or heater cores, automobile fuels tanks and the like, it has long been necessary to test the same for leaks before approving them for use. Various approaches have been taken to this leak testing, some of which are shown in the following U.S. Pat. Nos.: 2,874,566, 3,221,539, and 4,047,423. Despite these and other prior efforts, the need remains for leak detection which may be carried out in but a few seconds.
Heretofore leak testing using air as a testing medium required anywhere from five to 12 seconds for the air temperature in the chamber being tested to stabilize before the leak test was effected. Without this stabilization, temperature changes of the air in the chamber being tested would give a false reading. Such temperature changes could result from the adiabatic cooling effect of compressed air expanding as it is released into the chamber, or if the chamber walls were cooler or warmer than the air introduced into the chamber, heat transfers and consequent pressure changes would occur. With the requirement to increase production so that more parts could be tested in a given period of time, there has been the concurrent need to reduce the stabilization time.
Also, serious errors in leak detection based on the use of air as a test medium could arise in the production environment of a large manufacturing plant as a result of diurnal temperature variations which, in some instances, might be as much as 20.degree. F. in a 12-hour period. With this kind of variation, even a test taking less than one minute may permit a sufficiently significant temperature change in the test chamber, rendering wholly inaccurate a leak test based on air pressure change. Thus, there has been a need to neutralize these diurnal temperature effects which would cause changes in the temperature of the chamber being tested and distort the efficacy of the leak test.
Another error which could distort the accuracy of prior art leak testers might arise from the lack of uniformity of temperature of the parts being tested. For example, parts which have been waiting for some hours at room temperature to be leak tested might be cooler than parts coming directly from a parts washer, and yet if this temperature difference is not compensated for, there will be inaccuracies in the test results, viz., those parts which are warmer will tend initially to heat the air to the part temperature and thereafter when the test is being performed the air will be cooling as the part cools and create a pressure drop simulating a leak when a leak may not in fact exist. Thus, the need to compensate for such errors was an important problem to be solved in arriving at a satisfactory leak tester.
Because of the difficulties of leak testing structures, such as automobile radiators or fuel tanks and occasionally engine blocks using air as a test medium, such products are often tested in large waterbath type testing machines relying on visual detection of air bubbles to signal leaks. As such a method is slow and relies heavily on human eyesight, a faster and more reliable method has been needed for some time.